1. Field of the Invention
The present invention relates to novel alumina-based compositions, to processes for the preparation thereof, and to the use of such novel compositions as starting materials for the production of particular catalysts or catalyst supports.
This invention especially relates to novel alumina-based compositions having a high specific surface area provided by pores having an average diameter ranging from 10 nm to 100 nm.
2. Description of the Prior Art
Porous activated alumina has long been known to this art as a catalyst support, adsorbent, etc. In the field of heterogeneous catalysis and in particular of catalysis for the conversion of exhaust gases emanating from internal combustion engines, the alumina must have a high specific surface area which is stable to heat and, moreover, a porosity of suitable dimensions in order to permit, on the one hand, a good dispersion of the catalytic phase therein and, on the other, a good diffusion therethrough of the gases to be treated.
Numerous processes are also known to this art for the production of alumina in powder, gel or agglomerate form having a high specific surface area and possessing a substantially bimodal pore diameter distribution, namely, micropores having a diameter of less than or close to 10 nm and macropores having a diameter of more than 100 nm. For example, the porosity of an alumina agglomerate may be provided by addition of a pore generator (pore-forming agent) which decomposes during the heat treatment of the agglomerate, or by agglomeration processes of the filler/binder type, the filler being an alumina powder and the binder being an alumina gel. Processes are also known to this art for the production of alumina powder or gel having a high microporosity. However, the conditions under which these processes are carried out are very difficult to control.
In catalysis applications, the alumina is generally employed as a support on which catalytically active elements such as precious metals, molybdenum, vanadium, etc., and also other elements such as promoters, are impregnated. Exemplary thereof are the rare earths, which are conventionally used in the catalytic conversion of exhaust gases emanating from internal combustion engines as a stabilizer for the specific surface area of the alumina and/or as a promoter, for example by utilizing their redox properties, such as cerium.
Alumina is also used to produce a porous coating on a compact structure designated monolithic, is generally in honeycomb form and is shaped from either a refractory material such as a ceramic, or from metal. In this case, the catalytic compounds such as the precious metals or the other elements such as the promoters are impregnated onto the porous layer of the alumina, previously deposited onto the monolithic structure.
To date, these elements were impregnated on an alumina having specific morphological characteristics. These successive impregnations necessitate a heat treatment of the alumina which may influence its morphology. Moreover, the dispersion of the elements at the surface of the pores is not always homogeneous.